The present invention relates to the preparation of workpieces, and in particular relates to the cleaning of workpiece surfaces using a hydrogen-based plasma.
A fundamental step in the manufacturing of semiconductor devices, such as integrated circuits (ICs), is the process of forming electrical interconnections, or xe2x80x9ccontacts.xe2x80x9d The formation of a low resistance contact involves the steps of providing a semiconductor workpiece, such as a silicon wafer, cleaning the surface of the workpiece, selectively depositing a metal, such as titanium, on the surface, and thermally annealing the metal. Where the metal is titanium, the annealing process causes the formation of titanium silicide, which consumes some of the underlying silicon.
Unfortunately, as minimum circuit dimensions decrease, the use of metals for forming the electrical contacts becomes problematic. This is primarily because the resistivity of the metal-silicon (e.g., titanium silicide) contact increases dramatically when the size of contact (i.e., the xe2x80x9cline widthxe2x80x9d) is one micron or less. Compounding the problem, as line widths diminish below one micron, device junction depths decrease to just a few hundred angstroms. Since the formation of silicides consumes some of the underlying silicon, a reduction in the junction depth to a few hundred angstroms means that the integrity of the junction is at risk.
Use of the metal cobalt has been proposed as a solution to the above-described problems associated with titanium-based contacts, and is used in sub 0.25 micron manufacturing processes. However, the use of cobalt in forming contacts introduces additional problems. For example, cobalt does not react with silicon oxides or any of the other likely surface contaminants, such as water and C-F polymers. Consequently, the surface of the wafer prior to cobalt deposition must be far cleaner than what is necessary for other metal-silicon contacts, such as titanium silicide.
There are two techniques currently used in semiconductor manufacturing to clean workpiece surfaces prior to forming contacts using cobalt. One method is to clean the wafer in a variety of chemical solutions, including a final step of cleaning the wafer with a hydrofluoric (HF) dip. Though this approach is effective for many cleaning processes (especially those involving 0.5 micron technology and higher), HF is not as sufficiently reliable for sub 0.25 micron technology. Furthermore, this chemical poses significant health risks to operators and technicians. Moreover, workpieces to be processed must be transported from the HF dip tank to the deposition reactor. In this transportation step the workpieces are exposed to air, which oxidizes the exposed surface, thereby degrading device performance and reducing process tolerances.
The second workpiece cleaning surface method used prior to cobalt deposition involves sputtering the workpiece surface with argon ions. To be effective, the energy of the ions must be reasonably high. Unfortunately, use of such high-energy ions is problematic. For example, sputtering at such high energies can result in argon being incorporated into the silicon. Such ions can result in the generation of crystal defects as deep as several hundred angstroms. Other problems include erosion of the silicon itself, re-deposition of the sputtered materials, and the penetration of surface contaminants into the silicon.
The use of hydrogen plasma has been proposed as a method for cleaning surfaces. Since the chemical byproducts of hydrogen plasma are essentially gaseous, the cleaning process should be very effective. However, when the use of hydrogen plasma to clean wafers was studied, numerous problems emerged. For example, when using a parallel plate reactive ion etch (RIE) system, severe silicon erosion and the diffusion of hydrogen into the silicon resulted from the high ion energy generated in the source plasma. When a microwave-excited downstream plasma was used, the removal rate of the native oxide and other contaminants was significantly reduced due to the low energy of the hydrogen radicals and the reduction of radical concentration during the transportation from the source to the wafer surface.
The present invention relates to the preparation of workpieces, and in particular relates to the cleaning of workpiece surfaces using a hydrogen-based plasma.
A first aspect of the invention is a method of plasma cleaning a workpiece in a plasma-cleaning chamber having an interior region. The method comprises the steps of first, loading the workpiece into the plasma cleaning chamber interior region. The next step is pumping the plasma cleaning chamber interior region down to a pre-determined pressure, with hydrogen as the ambient gas. The next step is forming from the hydrogen gas a plasma having an ion density in the range of 1010 to 1013 cmxe2x88x923 and preferably greater than 1012 cmxe2x88x923, and an ion energy lower than 30 eV and preferably in the range from 10 to 15 eV. The last step is exposing the workpiece to the plasma for a predetermined time.
A second aspect of the invention is the method as described above, further including the steps, after the wafer is cleaned, of transferring the workpiece from the plasma cleaning chamber to a processing chamber, and then performing a process step to the workpiece. This process step may be, for example, depositing a metal.
A third aspect of the invention is an integrated workpiece processing apparatus for plasma cleaning a workpiece and then processing the workpiece. The apparatus comprises a first vacuum processing chamber adapted to plasma clean a workpiece with a plasma having a high ion density, low ion energy and low plasma potential. The first vacuum processing chamber includes a workpiece support fitted therein. The apparatus also includes a second vacuum processing chamber adapted to perform a process selected from the group consisting of CVD, PVD, sputtering, and etching of a workpiece. The second processing chamber is also fitted with a workpiece support. Further included in the apparatus is a vacuum transfer chamber connecting the first and second chambers. The transfer chamber is sized so that a workpiece may pass between the chambers. The purpose of the transfer chamber is to prevent the workpiece from being exposed to contaminants (i.e., oxygen or water vapor, etc.) after it has been cleaned in the first vacuum processing chamber.